Cis 1,4-polyisoprene rubber-rich, particularly natural rubber-rich rubber compositions, may be prepared, for example, by blending cis 1,4-polyisoprene rubber (e.g. cis 1,4-polyisoprene natural rubber), silica reinforcement, particularly precipitated silica, together with a bifunctional organosiloxane polysulfide coupling agent.
However, it is known that a reduction in various physical properties of natural rubber may occur if the rubber mixture has a relatively high mixing viscosity (e.g. Mooney viscosity), with an accompanying relatively high internal heat generation of the rubber mixture as it is being mixed under high shear conditions, thereby resulting in a relatively rapid internal temperature build up of the rubber mixture.
The inclusion of an alkoxysilane-containing organopolysulfide coupling agent, for example a bis(ω-alkoxysilylalkyl) polysulfide which contains an average of from about 2 to about 4 connecting sulfur atoms in its polysulfidic bridge, in a rubber mixture which contains precipitated silica (with hydroxyl groups such as, for example, silanol groups) and diene-based elastomer-containing rubber mixture can significantly increase the viscosity of the rubber mixture as it is being mixed under high shear conditions in an internal rubber mixer via an interaction of sulfur groups from its polysulfide moiety with the diene-based elastomer(s), a well known effect to those having skill in such art.
Accordingly, it is desired to mix such rubber mixtures more efficiently under high shear conditions in an internal rubber mixer.
Further, the addition of such alkoxysilane-containing polysulfide coupling agents may also release an alcohol during its mixing with the natural rubber-rich rubber composition as it reacts with hydroxyl groups (e.g. silanol groups) contained on the precipitated silica aggregates, a phenomenon also well known to those having skill in such art, which may be objectionable where release of alcohols, particularly volatile alcohols, may not be desired.
U.S. Pat. No. 6,239,203 relates to a method for improving the abrasion resistance of a cured rubber composition for a tire tread by dispersing a C12-C36 alcohol and sulfur throughout a dry, isolated, sulfur vulcanizable rubber followed by curing the resulting rubber composition. The rubber composition may contain natural rubber. Suitable alcohols for such purpose are indicated to be primary, straight chain, saturated alcohols such as, for example, 1-octadecanol.
In the description of this invention, rubber compound, sulfur-cured rubber compound, rubber composition, rubber blend and compounded rubber terms may be used interchangeably to refer to rubber (e.g. elastomer) which has been mixed with rubber compounding ingredients, unless otherwise indicated. The terms “rubber”, “elastomer” and “rubbery polymer” may be used interchangeably unless otherwise indicated. The terms “cured” and “vulcanized” may be used interchangeably unless otherwise indicated.
The term “phr” refers to parts by weight of an ingredient per 100 parts by weight of rubber in a rubber composition.
Such terms are well known to those having skill in such art.
The terms “dry” and “dry, isolated” conjugated diene rubber, including natural rubber (cis 1,4-polyisoprene natural rubber), relates to such rubber in its dry state instead of being in a solvent solution thereof or an aqueous latex thereof.